Light-emitting elements including semiconductor materials (hereinafter also referred to as LED chips) are small and capable of emitting bright colors with high power efficiency. LED chips are characterized by having long product life and low power consumption, and being robust to repeated on/off switching. As such, they are useful for lighting sources such as liquid crystal display backlights and lamps.
LED chips can be used for illumination devices which have a phosphor to change part of the light wavelengths from the LED chips and emit a mixture of light with the changed wavelength so that light of a color different from that of the LED can be emitted. Light with a non-changed wavelength from the LED can also be emitted.
Existing technology uses a violet LED chip having a peak at 400 nm or less as a light-emitting element. A structure containing a mixture of three types of phosphors in a polymer resin can be used in a wavelength converter. The wavelength converter can convert violet light to red, and can convert yellow to green and blue wavelengths, respectively so that white light can be emitted. Europium (Eu)-containing phosphors have been actively developed to provide phosphors capable of being used in combination with an LED chip as a light-emitting element having a peak wavelength at 400 nm or less and capable of emitting color of yellow to green (hereinafter referred to as “yellow to green color”).
A phosphor represented by Sr2−x−yBaxEuySiO4 and the production of the phosphor with a (Sr+Ba+Eu)/Si molar ratio of 2, where the (Sr+Ba+Eu)/Si molar ratio is a sum of the molar ratios Sr/Si, Ba/Si and Eu/Si has been used to produce luminous efficiency of the illumination device. However, the luminous efficiency of the illumination device produced with this phosphor is still low, and therefore, an increase in luminous efficiency is needed.
Therefore, there is a need for a phosphor operable to efficiently absorb ultraviolet or visible light and emit visible light with longer wavelength.